1. Field of the Invention
The present invention relates to a compression gas puffer type circuit breaker and, in particular, relates to an improvement in the arrangement and cooling structure of a magnetic body for controlling an inter electrode transient recovery voltage which is provided in such a compression gas puffer type circuit breaker so as to reduce the interrupting duty thereof in the event of a short line fault.
2. Conventional Art
Due to an increase in voltage level and power transmission capacity of a power transmission system such as introduction of an UHV power transmission in association with recent increasing demand of electrical power, a current to be interrupted by a circuit breaker in an event of grounding fault in a power transmission system is continuously increasing. Further, as a result of the conditions imposed by the locations of such sub-stations and switching stations in these days, down sizing of the gas circuit breakers has become an indispensable technical tasks. In order to resolve such a technical task, an improvement in current interrupting performance of such circuit breakers is proposed to reduce current interruption points in the circuit breakers and to increase current interrupting capacity per one current interrupting point.
Under these circumstances, the increase of the power transmission capacity causes an increase in the current to be interrupted and in the variation rate of the interrupting current in the event of a short line fault, which also increases the rate of rise of transient recovery voltage in a circuit breaker, thereby an increase in current interrupting duty is required for the circuit breaker.
One solution for such problems is disclosed, for example, in JP-A-3-190028 in which the variation rate of the interrupting current is reduced with a provision of a magnetic body disposed coaxialy with a stationary arc contact of a compression gas puffer type circuit breaker, to thereby suppress the increase in the rate of rise of the transient recovery voltage in the circuit breaker.
Because the magnetic body is disposed in a manner so as to surround the stationary arc contact, in the event of a short line fault an interrupting current flows through the movable and stationary arc contacts via the current arc therebetween, the magnetic fluxes generated by the interrupting current crosslink the magnetic body and the self inductance of the magnetic body begins to increase immediately before the interrupting current reaches the zero point. As a result, the variation rate of the interrupting current is reduced and the rate of the rise of transient recovery voltage between electrodes of the circuit breaker is also reduced in proportion to the variation rate of the interrupting current, thereby the current interrupting capacity of the circuit breaking portion equivalently increases and the current interrupting performance of the circuit breaker is improved.
However, in the above explained prior art circuit breaker, the magnetic body is directly exposed to a heated gas which is exhaused at a high speed from an insulation nozzle and flows into a contact base surrounding the stationary arc contact. Further, when the magnetic body is excited by an interrupting current, the temperature of the magnetic body is raised because of heat generation by the magnetic body itself due to electric power losses such as hysteresis loss and eddy current loss. Still further, a current arc is generated at a top of the stationary arc contact which the magnetic body surrounds and the stationary arc contact is heated thereby, and the temperature of the magnetic body is further raised because of thermal conduction through the conductor constituting the stationary arc contact.
The magnetic body for controlling the transient recovery voltage is generally formed of amorphous or ultra fine crystalline soft magnetic materials such as ferrite and amorphous alloys having Curie temperatures Tc are at most about 570.degree. C. and if the temperature of the magnetic body execeeds its Curie temperature, the magnetic body is completely demagnetized. Further, when the temperature of the magnetic body exceeds about 100.degree. C., the saturation magnetic flux density of the magnetic body decreases and the magnetic coercive force increases, accordingly the amount of retainable magnetic flux in the magnetic body decreases and the electric power loss therein remarkably increases.
Therefore, with the conventional structure an extreme temperature rise of the magnetic body was unavoidable and an intended performance of the magnetic body could not be obtained because of the deterioration of the magnetic properties thereof.
Further, with the conventional structure the stationary arc contact only once passes through the magnetic body because of the structure thereof such that the number of effective magnetic flux linkages with the magnetic body is inherently limited. As a result, in order to permit a predetermined amount of magnetic fluxes to be accepted, it is indispensable to increase the volume of the magnetic body, thereby the electric power loss in the magnetic body increases and the size and weight increase of the magnetic body becomes unavoidable in accordance with the volume increase thereof.